Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or “bubble jet”), uses electroresistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electroresistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices. Piezo-electrically actuated devices do not impose such severe limitations on the liquids that can be jetted because the liquid is mechanically pressurized.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648; and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Pat. No. 5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires mono-dispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission is needed that can be used with a broad range of liquid formulations. Apparatus and methods are needed that combine the advantages of microelectronic fabrication used for thermal ink jet with the liquid composition latitude available to piezo-electro-mechanical devices.
A DOD ink jet device that uses a thermo-mechanical actuator was disclosed by T. Kitahara in JP 2,030,543, filed Jul. 21, 1988. The actuator is configured as a bi-layer cantilever moveable within an ink jet chamber. The beam is heated by a resistor causing it to bend due to a mismatch in thermal expansion of the layers. The free end of the beam moves to pressurize the ink at the nozzle causing drop emission. Recently, disclosures of a similar thermo-mechanical DOD ink jet configuration have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,234,609; and 6,239,821. Methods of manufacturing thermo-mechanical ink jet devices using microelectronic processes have been disclosed by K. Silverbrook in U.S. Pat. Nos. 6,254,793 and 6,274,056.
Thermo-mechanically actuated drop emitters are promising as low cost devices which can be mass produced using microelectronic materials and equipment and which allow operation with liquids that would be unreliable in a thermal ink jet device. In addition, apparatus and methods of operating liquid drop emitters so as to usefully generate drops having substantially different drop volumes would be highly desirable. Such apparatus and methods would allow a single drop emitter to provide different levels of the liquid per drop firing cycle. In ink jet printing this capability may be used to generate multiple image gray levels while preserving the printing speed associated with binary printing. The gray level printing capability of a single ink drop emitter may allow a printing system to be designed with fewer jets to achieve lower overall system cost or, alternatively, may be configured to achieve higher net printing speeds of gray level images at apparatus costs similar to a binary level printing system.
Some methods of emitting different ink drop volumes from drop-on-demand ink jet printheads have been disclosed and used previously. Use of fluid resonances for such purpose is known for piezoelectric drop-on-demand ink jet devices. In these known methods, the resonance of the ink meniscus at the nozzle, driven by surface tension effects, or the Helmholtz resonance of the ink chamber, driven by compliance effects, is used to change the volume or number of emitted drops. Tence et al. in U.S. Pat. No. 5,689,291 employ waveforms that drive piezoelectric transducers with spectral energy concentrations at frequencies associated with modal resonances of ink in the ink jet printhead orifices. Exciting different resonance modes of the ink meniscus causes the emission of different drop sizes.
DeBonte et al., in U.S. Pat. No. 5,202,659 disclose a method of operating a piezoelectric printhead using the dominant resonant frequency of the ink jet apparatus. This dominant resonance is described as the Helmholtz resonance of an individual jet chamber, which is excited by actuating the piezo transducer to first expand the jet chamber, waiting the resonance period, and then contracting the chamber to reinforce this resonance. This excitation process is repeated for multiple cycles to generate multiple merging drops for printing spots having different sizes.
Paton et al., in U.S. Pat. No. 5,361,084 disclose a method of multi-tone printing using a piezoelectric DOD printhead having elongated ink chambers and sidewall actuators, wherein an individual jet is excited using a packet of pulses so as to excite a longitudinal acoustic resonance in the jet channel that causes the emission of a number of discrete drops. Lee et al., in U.S. Pat. No. 4,513,299 disclose a similar use of acoustic resonance of the ink channels of a piezoelectric ink jet printhead.
The piezoelectric transducer used in a piezoelectric printhead may be driven to both compress and expand the ink fluid chamber, thereby allowing the ink meniscus at the nozzle to be pushed out or pulled inward. A variation in emitted drop volume may be achieved by manipulating the meniscus position and velocity by a sequence of compressive and expansive electrical pulses. Apparatus and methods of operating a piezoelectric drop-on-demand inkjet printhead in this fashion have been disclosed by S. Sakai in U.S. Pat. No. 5,933,168 and by Horii, et al., in U.S. Pat. No. 6,095,630.
Apparatus and methods of operating a thermal ink jet drop-on-demand printhead to create multiple drop volumes also have been disclosed. For example, Bohorquez, et al., in U.S. Pat. No. 5,726,690, describe a method of operating a thermal inkjet printhead that includes changing the pulse width of the driving electrical pulse, increasing the applied energy and thereby resulting in the emission of larger drops for larger energy inputs. Drop volumes that range in magnitude approximately 16% are disclosed.
Larger drop volume changes are reported for thermal ink jet apparatus and methods that are configured so that different areas of heater resistor can be energized. For example, Ishinaga, et al., in U.S. Pat. No. 5,880,762 discloses an apparatus having a plurality of heat generating resistors per ink nozzle chamber. The plurality of heat generating resistors are driven independently to cause the emission of several different drop volumes. J. Wade, in U.S. Pat. No. 6,318,847, discloses a segmented area heater resistor configuration that may be energized to generate a range of vapor bubble volumes causing the emission of differently sized drops.
Thermo-mechanical actuators are substantially smaller in scale than the piezoelectric actuators used in ink jet printheads and have mechanically different resonant behaviors. Thermo-mechanical actuators are more complex to fabricate than thermal ink jet heater resistors and, therefore, more difficult to construct in a multiple-actuator per jet configuration in analogous fashion to the disclosed thermal ink jet apparatus above noted. Apparatus and methods that generate variable drop volumes are needed which are adapted to the unique physical configurations, behaviors and capabilities of thermo-mechanical actuators.